Commercial warewashers vary significantly in their design and manner of use, but all include an open-topped water-containing tank. The design is frequently dictated by the kitchen environment in which the warewasher is used and the volume of ware to be washed. Typically, the tank has pre-heated water recirculated through wash arms under pressure from a pump. To do an effective washing job, the water temperature must be maintained at an approximate temperature of about 150 degrees F. in a high-temperature machine (one using a fresh final rinse at 180-195 degrees F.) or at about 140 degrees F. in a low-temperature machine (one utilizing a final rinse mixture of fresh water and sodium hypochlorite for sanitizing, also at about 140 degrees F.). Industry has taken a number of different approaches in seeking the most cost-effective, energy-efficient type of warewasher, consisting of electricity, steam and gas in descending order of sales volume. The most common usage is of electrical heating elements immersed in the tank water, for two primary reasons. First and most important is the ease of installation for the end user. Electrical outlets are commonly available in a commercial food establishments, thus the unit can be merely plugged in and operated upon delivery. Secondly, electricity is essentially one hundred percent thermally efficient, because the elements are totally immersed in the tank water. Cost of operation is electricity's primary disadvantage, however, since a warewasher is used mainly during and just after mealtimes, when the larger electrical load during peak demand for electricity dictates higher utility rates to the end user. Another disadvantage is that electrical units are also slower in response time than gas.
While the cost rate of gas remains constant and is lower priced than electricity for the amount of energy provided, gas usage has nevertheless made little headway in the warewasher industry. One reason is the need to install gas lines to the warewasher. Even though gas may be available in a kitchen, it is far easier to plug in an electrical unit than install additional gas piping for a new piece of equipment which operates on gas. Much more importantly, however, is the fact that the typical blue flame gas-fired warewashers of the past have been relatively inefficient. The burners were placed below the wash or rinse tank and the flame would function much like heating a pot over a stove, where energy is lost around the pot and escapes up a vent with combustion products which are somewhat air-polluting. Many such gas-fired warewashers, which represent as little as five percent of total warewashers sales of our assignee, have a heat transfer efficiency as low as fifty percent. This necessitates that burners with higher BTU (British Thermal Unit) ratings than is required to get effective heat transfer be used, since so many BTU's are lost to vent, tending to pollute the atmosphere as well. Thus, the lower-price, constant gas rates are, in effect, more than offset by the waste which occurs with such conventional gas-heated warewashers, due to the very low efficiency of conventional blue flame gas heating.